Add support for CommandListeners

[sokomishalov]

It would be great to have a handler API of executing commands.
I think there are a lot of possible use cases.

For example, I want to log all executing commands (input and output).
Current logging is not very human-friendly, actually :)
My temporary solution looks like this, but I don't think it's quite elegant:
https://gist.github.com/SokoMishaLov/207c8082594685daa44cbd60127960a8

Thanks!

[mp911de]

Thanks. We could introduce an API along the lines of:

public interface CommandListener {
    /**
     * Listener for command started events.
     *
     * @param event the event
     */
    default void commandStarted(CommandStartedEvent event) {
    }

    /**
     * Listener for command completed events
     *
     * @param event the event
     */
    default void commandSucceeded(CommandSucceededEvent event) {
    }

    /**
     * Listener for command failure events
     *
     * @param event the event
     */
    default void commandFailed(CommandFailedEvent event) {
    }

}

that get notified upon command events from CommandHandler. Since support for metrics and tracing are configured on ClientResources, it makes sense to put command listeners there, too.

Command events would typically hold the command type, arguments and the output (for success) and the error/exception for the failed case.

We also need to consider that commands may get canceled (i.e. connection closed) from the endpoint implementation which would ultimately call commandFailed.

[sokomishalov]

That's almost exactly what I expected to see.
I think the command event should also hold context/correlation/whatever to correlate events.

[mp911de]

Do you have an example of how you would like to attach data to a command or how the API for the context should look like?

[sokomishalov]

I think it should look like typical key-value storage.
I suppose that r2dbc-proxy (well-known to you btw) has a nice API for similar purposes.

https://github.com/r2dbc/r2dbc-proxy/blob/main/src/main/java/io/r2dbc/proxy/core/QueryExecutionInfo.java#L180
https://github.com/r2dbc/r2dbc-proxy/blob/main/src/main/java/io/r2dbc/proxy/core/ValueStore.java

[FitBBC]

No place to configure CommandListener found in ClientResources

[mp911de]

Command listeners are configured via RedisClient/RedisClusterClient